In automation technology, especially in process automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors, which are integrated, for example, into fill level measurement devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., and which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, redox potential, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, or the fill level in a container, can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. In connection with the invention, understood to be field devices are thus also remote I/Os, radio adapters, and, generally, electronic components, which are arranged at the field level. A large number of such field devices are produced and sold by the firm, Endress+Hauser.
A frequently required criterion for field devices is explosion protection, since field devices are often applied in explosion-endangered regions. Accordingly, they must be designed in accordance with pertinent explosion protection standards. These are established, for example in Europe, by the family of standards, EN 60079. Especially, these standards relate to the housings of the different field device types, since the character of the housing decisively influences the explosion protection properties of the field device. In order to conform to the standards, it is, among other things, advantageous, when the housing has an as closed as possible form of construction. Under this point of view, electrical and mechanical cable passageways through the housing-wall, for example, for electrical contacting or for information exchange, represent a major concern. As a result, it makes sense to design the structure, as much as possible, so as to avoid these cable passageways.
Corresponding structural specifications are, however, difficult to put into practice, when, for example, the serviceability of field devices by service personnel is considered. Thus, often, interfaces must be present for read-out of data, for configuring the device and for inputting parameters. Besides graphic display, this means also input elements on the housing outer surface, such as, among other things, pushbutton switches.
Known from the state of the art are pushbutton switches, via which an information input into the housing interior of field devices can be performed without electrical or mechanical passageway.
Thus, in U.S. Pat. No. 5,353,200, a housing for a field device is provided, in the case of which the information input occurs via one or more pushbutton switches. In such case, the housing exterior is embodied as an at least partially elastic layer, which in the case of contact at the appropriate location actuates a pushbutton switch located in the housing. This variant of embodiment offers very favorable conditions, especially as regards electromagnetic compatibility, also known as EMC. However, a non-solid embodiment of the housing exterior is unsuitable from an explosion protection point of view, since it cannot assure a sufficient defense.
In contrast therewith, magnetic switches offer the opportunity of information input through a solid housing wall, without requiring a passageway or an elastic portion in the housing wall. Thus, for example, buttons with integrated magnet bodies can be used, which are positioned and movable in defined manner with reference to the housing, in order to actuate a Hall-sensor in the interior of the housing by means of a magnetic field.
Offenlegungsschrift DE102004036324A1 discloses a pushbutton switch for a measurement transmitter, in the case of which a support body, which contains a magnet body in asymmetric position, can be inserted in different orientations in a blind hole, wherein the blind hole is directed toward the interior. Depending on orientation of the magnet body in the blind hole, a Hall sensor in the interior of the housing is variably influenced by the magnet body. Via the the particular orientation of the magnet body, either of two manners of operation of the measurement transmitter can be selected.
European Patent EP 0383 823 B1 discloses a measurement transmitter, in the case of which the zero-point and the slope can be set by means of magnetic switches. The magnetic switches have, in each case, a magnet, which is movable in a blind hole, in order to actuate a reed-contact, which is arranged on the housing-interior side of the blind hole.
Common to the described magnet switches is that the housing must have an inwardly directed, blind hole, in order to be able to function. At the same time, a comparatively large pushbutton stroke is required, in order unequivocally to be able to identify the pressed-state. In this way, a sufficiently large dimensioning of the interior as well as a rather constrained arrangement of the magnetic switch located in the housing interior are required. These requirements affect the compactness of the field device negatively. Moreover, it becomes difficult to offer platform designs of different field devices based on the same housing-type.